1 . Field of the Invention
The present invention generally relates to an X-ray imaging method and an X-ray imaging system capable of discriminating an X-ray contrast image from a non-contrast image. More specifically, the present invention is directed to an X-ray imaging method and an X-ray imaging system for judging whether or not an X-ray contrast image of a biological body under medical examination can be acquired during X-ray imaging operation by checking temporal variations contained in averaged values of image concentration with respect to subdivided X-ray image regions.
2. Description of the Prior Art
In the X-ray imaging systems, an X-ray contrast medium is injected via a blood vessel into an imaging region of a biological body under examination while inserting a catheter, a guide article, a wire and the like into the blood vessel thereof, and projecting X-rays to these imaging region and blood vessels so as to acquire a series of X-ray images.
In such X-ray imaging system with employment of the X-ray contrast medium, there are the following difficulties. That is, since the X-ray contrast medium quickly disappears from such an imaging region after injection operation of the X-ray contrast medium, it is absolutely necessary to correctly detect insertion timings of X-ray contrast medium in order to acquire X-ray images of the imaging region with which the X-ray contrast medium is mixed (so-called "contrast images").
Various ideas to detect such injection timings of the X-ray contrast medium have been proposed.
As to the first timing detection method, the variations in the averaged value of image concentration, or density for the overall imaging region are processed to detect such an insertion timing of the X-ray contrast medium. This conventional detecting method is realized based upon such a phenomenon that when the X-ray contrast medium is injected into the imaging region, the averaged value of the image concentration with respect to the overall imaging region is lowered. The first detecting method is known from, for instance, Japanese KOKAI (DISCLOSURE) patent application NO. 64-58243 opened in 1989.
As to the second timing detection method, upon injection of the X-ray contrast medium an operator immediately turns off the X-ray imaging switch, and the X-ray image acquired just before the imaging switch is turned off is held (so-called "last image holding method"). This second detecting method is described in, for example, Japanese KOKAI patent application NO. 60-185481 (1985).
In the first detecting method, however, there is such a problem that since the averaged value of image concentration for the entire imaging region is utilized so as to detect the injection timings of the X-ray contrast medium, erroneous detecting operation may happen to occur if a biological body under medical examination is positionally shifted or moved.
Furthermore, the second detecting method has another problem that since turning off the X-ray imaging switch must be carried out in synchronism with the injection operation of the X-ray contrast medium, very cumbersome operations are necessarily required of any operators.
The present invention has been made in an attempt to solve the above-described various problems, and therefore has an object to provide an X-ray imaging method and an X-ray imaging apparatus capable of discriminating a contrast image from a non-contrast image even when a biological body under medical examination is positionally shifted.
To achieve the above-described object and features of the present invention, an X-ray imaging method is characterized by comprising the steps of:
acquiring a plurality of X-ray images (X.sub.i) of a biological body (20) under medical examination;
subtracting at least two sets of the X-ray images (X.sub.i :x.sub.i-l) with each other acquired at temporally different timings to obtain a subtraction image (G.sub.i);
subdividing one subtraction image (G.sub.i) into a plurality of image blocks (B.sub.00 :B.sub.33);
calculating an averaged value of image concentration with respect to each of the image blocks (B.sub.00 :B.sub.33), thereby obtaining a plurality of averaged values for the subdivided image blocks (B.sub.00 :B.sub.33);
calculating a difference between at least two averaged values of image concentration with respect to the image blocks (B.sub.00 :B.sub.33) acquired within substantially same temporal timing period; and,
judging whether or not the X-ray image (X.sub.i) is acquired under such a condition that an X-ray contrast medium appears therein based upon the difference in said two averaged values of image concentration.
Furthermore, an X-ray imaging system (100:200), according to the present invention, comprises:
means (1:3) for producing a plurality of X-ray images (X.sub.i) of a biological body (20) under medical examination;
subtraction means (8) for subtracting at least two sets of the X-ray images (X.sub.i :X.sub.i-1) with each other produced at temporally different timings to obtain subtraction image (G.sub.i);
averaging means (10) for obtaining an averaged value of image concentration for each of image blocks (B.sub.00 :B.sub.33) subdivided from one subtraction image (G.sub.i), thereby obtaining a plurality of averaged values for the subdivided image blocks (B.sub.00 :B.sub.33);
calculating means (11:12) for calculating a difference between at least two averaged values of image concentration with respect to the image blocks (B.sub.00 :B.sub.33) acquired within substantially same temporal timing period; and,
judging means (14:50) for judging whether or not the X-ray image (X.sub.i) is acquired under such a condition that an X-ray contrast medium appears therein based upon the difference in said two averaged values of image concentration.